I've been having a play with Cambam, and it does have potential.

From the hour or so play I had on it, it looks like it will be able to produce the majority of the external machining code (i.e. no boring/drilling/i.d. turning) I need for now, and for what internal machining I need, I should get away with the wizards/hand writing the g-code.

I've now just got to decide how to mount the drill bits that I'll be needing...